subway sandwiches are probably the cheapest, quickest, and healthiest 'fast food' that you might tolerate eating on a daily basis. you can get a decent sandwich for ~$4.
Without the cheese, bacon and sauce... But even then, there is way too much sugar in the bread.
You can ask the person behind the counter to scrape some of the bread out from the middle before they start adding meats and veggies. I do it all the time and it makes the sandwich taste better too (due to the higher flavor-to-bread ratio)